1. Field of the Invention
The present invention relates to a piezoelectric power generating element, and a method of generating electric power using the same.
2. Description of Related Art
Vibration power generation is an electric power generation method in which mechanical energy inherent in mechanical vibration is converted into electrical energy. Vibration power generation utilizes electromagnetic induction, electrostatic induction, or the piezoelectric effect, for example. Among these, vibration power generation utilizing the piezoelectric effect (piezoelectric power generation) exhibits high power density. This is because an element for piezoelectric power generation (a piezoelectric power generating element) has a simple structure and therefore can be reduced in size. A piezoelectric power generating element (a vibration power generating element) is an element that converts energy of mechanical vibration applied to a piezoelectric layer into electrical energy by the piezoelectric effect so as to generate electric power.
FIG. 11 and FIG. 12 show conventional piezoelectric power generating elements.
The element shown in FIG. 11 comprises a fixed-free type (cantilever-type) vibration beam 102. The vibration beam 102 has a strip shape. One end (fixed end) 103 of the vibration beam 102 is fixed to a frame 104. The other end 105 of the vibration beam 102 is not fixed to the frame 104. The end 105 is a free end. The end 105 can vibrate in a direction perpendicular to the main surface of the vibration beam 102. A power generation layer 101 is provided on one of the surfaces of the vibration beam 102. The power generation layer 101 comprises a piezoelectric layer and a pair of electrodes that sandwich the piezoelectric layer therebetween. When external vibration is applied to the element shown in FIG. 11, the vibration beam 102 vibrates in response to the vibration applied. This vibration deforms the piezoelectric layer included in the power generation layer 101. This deformation generates, based on the piezoelectric effect, a potential difference between the pair of electrodes that sandwich the piezoelectric layer therebetween. The power generation layers 101 may be provided on both of the surfaces of the vibration beam 102. Apart of the power generation layer 101 may also be provided on the surface of the frame 104. The element shown in FIG. 11 is disclosed in JP 09 (1997)-205781 A, for example.
The element shown in FIG. 12 comprises a fixed-fixed type (bridge-type) vibration beam 201. The vibration beam 201 has a strip shape. Both ends 202 and 203 of the vibration beam 201 are fixed to the frame 104. A power generation layer 101 is provided on one of the surfaces of the vibration beam 201. When external vibration is applied to the element shown in FIG. 12, the vibration beam 201 vibrates in response to the vibration applied. The amplitude of the vibration is maximum at the center portion 204 of the vibration beam 201. This vibration deforms a piezoelectric layer included in the power generation layer 101. This deformation generates, based on the piezoelectric effect, a potential difference between the pair of electrodes that sandwich the piezoelectric layer therebetween. The element shown in FIG. 12 is disclosed in JP 3790255 B1, for example.
In the element shown in FIG. 11, the vibration beam 102 is a cantilever-type beam. Therefore, the vibration of the vibration beam 102 is less restricted. The less restriction of the vibration allows a relatively large amount of electric power to be generated. On the other hand, the vibration beam 102 is susceptible to damage from excessive external vibration.
In contrast, in the element shown in FIG. 12, the vibration beam 201 is a bridge-type beam. The bridge-type vibration beam 201 is resistant to damage from external vibration. On the other hand, since both of the ends of the vibration beam are fixed, the vibration amplitude of the beam is smaller, and the amount of electric power generated tends to be small accordingly. JP 3939737 B1 discloses a technique for increasing the vibration amplitude of the bridge-type vibration beam. Specifically, in the element disclosed in JP 3939737 B1, the vibration beam is previously bent and held in a bistable state. In this element, the vibration beam vibrates between two stable states alternately. The amplitude of this vibration is larger than the amplitude that can be presented by the vibration beam 201 shown in FIG. 12.
In addition to JP 09 (1997)-205781 A, JP 3790255 B1, and JP 3939737 B1 above, JP 2006-246688 A and JP 2007-026804 A may be related to the present invention. JP 2007-026804 A does not disclose a piezoelectric power generating element. JP 2007-026804 A discloses a high-frequency micromachined switch. The piezoelectric power generating element is based on the technology relating to the vibration of a beam. The high-frequency micromachined switch is based on the technology for holding a beam as a switch in the OFF or ON position, that is, for switching the beam between specified two modes to hold it in one of the modes. No consideration is given to the vibration of the beam in the high-frequency micromachined switch. The technology on which the piezoelectric power generating element is based is completely different from the technology on which the high-frequency micromachined switch is based.